Discount code not been applied

Hi, yesterday I contacted Dell customer service to get a discount code which was provided by email after a telephone conversation. I purchased a laptop on the Dell website and entered the code on the correct place, but after checking my invoice I realized that the discount wasn't applied. The only one which was applied was a delivery discount.

I would like to know why the discount doesn't appear on my invoice being a price completely different to the one I initially show on the website.

I contacted the customer service today but the girl I spoke with looked completely lost and didn't help me at all. I'm wondering why a company with a minimum of reputation has a really poor customer service